1. Technical Field
The disclosure relates to a pilot selection method for different types of base stations in a heterogeneous wireless communication network, a wireless communication system and a base station thereof.
2. Related Art
In OFDMA wireless communication system, pilot patterns for different numbers of transmission streams have been completely defined in several wireless communication standards such as 3GPP LTE and IEEE 802.16m, and associated pilot selection methods have also been proposed. The transmission of pilot sub-carriers in downlink is necessary for enabling channel estimation, measurements of channel quality indicators such as the signal-to-interference ratio (SINR), frequency offset estimation and so forth. In order to avoid channel estimation error induced by pilot collision which often occurs at cell edges, interlaced pilot patterns are used in cellular wireless communications.
FIG. 1A is a schematic diagram illustrating a cluster 10 of three cells in a cellular wireless communication network. Referring to FIG. 1A, the cluster 10 is a common grouping of three cells 110, 120 and 130 all being within coverage of a base station tower 140. Cell identifications (ID) 111, 121 and 131 enclosed in solid lines respectively represent cell IDs for cells 110, 120 and 130; segment ID 113, 123 and 133 enclosed in dashed lines respectively represent segments IDs for cells 110, 120 and 130. The cluster 10, figures of cell ID and segment ID shown in FIG. 1A are just for an exemplary example.
FIG. 1B is a schematic diagram illustrating a rank-2 interlaced pilot patterns in IEEE 802.16m standard, where the rank-2 pilot patterns refer to a situation where a base station transmits two streams of data through at least two antennas. FIG. 1B illustrates three different pilot pattern sets P0, P1 and P2. Each of the pilot pattern sets P0, P1 and P2 contains 6 OFDM symbols, in which each of the OFDM symbols contains 18 contiguous sub-carriers. The resource elements with figures of 1 and 2 inside (such as a block 150 enclosed in a dashed line) in the resource unit are just an example indicating where the pilots appear in the pilot pattern set P0. Interlaced pilot patterns are generated by cyclic shifting the base pilot patterns (i.e., pilot pattern set P0 in FIG. 1B). The interlaced pilot patterns are used by different advanced base stations (ABS) for one and two streams. The interlaced pilot patterns for one stream (i.e., rank-1) are shown in FIG. 1C.
FIG. 1C is a schematic diagram illustrating a rank-1 interlaced pilot patterns in IEEE 802.16m standard, where the rank-1 pilot patterns refer to a situation where a base station transmits just one stream of data through at least an antenna. FIG. 1C illustrates three different interlaced pilot pattern sets IP0, IP1 and IP2. The interlaced pilot pattern set IP0 includes two different interlaced pilot pattern streams IPS0 and IPS1; the interlaced pilot pattern set IP1 includes two different interlaced pilot pattern streams IPS2 and IPS3; the interlace pilot pattern set IP2 includes two different interlaced pilot pattern streams IPS4 and IPS5. Each of the interlaced pilot pattern streams IPS0, IPS1, IPS2, IPS3, IPS4 and IPS5 contains 6 OFDM symbols, in which each of the OFDM symbols contains 18 contiguous sub-carriers. The resource element with a figure of 1 inside (such as a block 152 enclosed in a dashed line) in the interlaced pilot pattern stream IPS0 is just an exemplary example indicating where the pilots appear in the interlaced pilot pattern stream IPS0.
As shown in FIG. 1C, six of interlaced pilot patterns (i.e., interlaced pilot pattern streams) can be used for one data stream. The index of the pilot pattern set used by a particular ABS with IDcell=k is denoted by pk. The index, pk, of the pilot pattern set is determined by the IDcell according to the following equation (1).
                              p          k                =                  floor          ⁡                      (                          k              256                        )                                              equation        ⁢                                  ⁢                  (          1          )                    
In IEEE 802.16m, two conventional pilot selection schemes have been proposed for support of macrocells. One is proposed based on the cell-ID and mobile station ID (STID), and the other is proposed based only on the cell-ID. The rank-1 pilot patterns of the first pilot selection scheme are similar as shown in FIG. 1C. The associated rank-1 pilot selection is proposed as function of cell-ID, k and STID, m, as the follow equation (2):
                                          p            k                    =                      floor            ⁡                          (                              k                256                            )                                      ⁢                                  ⁢                                            s              m                        =                                          mod                ⁡                                  (                                                            m                      +                      k                                        ,                    2                                    )                                            +              1                                ,                      m            =            0                    ,          1          ,          2                                    equation        ⁢                                  ⁢                  (          2          )                    
In equation (2), the index of the pilot pattern set is denoted by pk for a particular ABS with IDcell=k, and sm stands for a pilot stream with a station ID m in a particular pilot pattern set. FIG. 1D is a schematic diagram illustrating a cell deployment scheme. The cell deployment scheme 16 includes a plurality of clusters consisting of three cells in each of the clusters as similar to that of FIG. 1A, where a cluster 160, as an example, includes a base station tower 162 covering three cells with cell ID as 3, 259 and 515 respectively. The first pilot selection scheme can be applied in the cell deployment scheme 16.
The rank-1 pilot patterns of the second pilot selection scheme are similar as being shown in FIG. 1C. The associated rank-1 pilot selection is proposed as a function of the cell-ID, k, only as the following equation (3):
                                          p            k                    =                      floor            ⁡                          (                              k                256                            )                                      ⁢                                  ⁢                              s            k                    =                      mod            ⁡                          (                              k                ,                2                            )                                                          equation        ⁢                                  ⁢                  (          3          )                    
In the equation (3), the index of the pilot pattern set is denoted by pk for a particular ABS with IDcell=k, and sk stands for a pilot stream in a particular pilot pattern set. FIG. 1E is a schematic diagram illustrating another cell deployment scheme. The cell deployment scheme 18 includes a plurality of clusters consisting of three cells in each of the clusters similar to that shown in FIG. 1A. A cluster 184, as an example, includes three cells with cell ID of 13, 269 and 525; another cluster 182, as another example, includes three cells with cell ID of 16, 272 and 528. The second pilot selection scheme can be applied in the cell deployment scheme 18.
The above-described pilot selection methods are just for a single transmission link, such as in the macrocell transmission. However, there cane be different types of advanced base station (ABS) other than the macrocell base station. For example, in IEEE 802.16m, there are specific ABS types such as macrocell advanced BS (ABS), macro Hotzone ABS, or femtocell ABS. The ABS types may be categorized into macrocell ABS and non-macrocell ABS by hard partition with 258 sequences (86 sequences per segment multiplied by 3 segments) dedicated for macrocell ABS. Moreover, the non-macrocell ABS is classified in a hierarchical structure. Cell type of non-macrocell ABS is partitioned as a public ABS and a closed subscriber group (CSG) femtocell ABS. The public ABS can be further categorized into different types such as: a hotzone ABS, a relay ABS, an open subscriber group (OSG) femtocell ABS and so forth. The CSG femtocell ABS can be further categorized such as: a CSG-closed ABS and a CSG-open ABS.
There is currently no method proposed on the pilot selection for support of the above heterogeneous wireless communication networks. Different types of transmission networks, such as the macrocell and non-macrocell networks, in which one is overlapped by the other, form a typical heterogeneous network deployment such as a macro-femto (i.e., macrocell-femtocell) heterogeneous network. In the macro-femto heterogeneous network, interference can easily occur because data can be transmitted across two different transmission links. Therefore, it is an important issue to find an effective and efficient pilot selection scheme so as to ensure a more reliable data transmission in such heterogeneous wireless communication network.